Pulsed lasers are used in wide variety of applications ranging from signal sources in telecommunications systems to optical sources in sensing and measuring equipment. Q-switched lasers, for example, provide high power, short duration pulses for optical sensing functions, optical time domain reflectometry, and the measurement of nonlinearities in optical fibers. Illustratively, Q-switched lasers are capable of generating peak pulse powers of the order of a few hundred watts or more at repetition rates in the tens of kilohertz range. Pulse durations of a about 1-100 nanoseconds are typical. Mode-locked lasers, on the other hand, may serve as high speed (e.g., multi-gigabit) signal sources in telecommunication systems, particularly soliton transmission systems. As such, the mode-locked laser may generate peak pulse powers of a few hundred milliwatts at repetition rates in excess of 10 GHz. Pulse durations of a few picoseconds are typical.
Q-switched and mode-locked lasers have been extensively reported in the scientific literature. Two basic structures have been successfully demonstrated: a fiber laser ring topology of the type described by F.Fontana et al. in U.S. Pat. No. 5,381,426 issued on Jan. 10, 1995 and a Fabry-Perot (FP) fiber laser configuration of the type shown in U.S. Pat. No. 5,450,427 granted to M. E. Fermann et al. on Sep. 12, 1995. Both of these structures suffer from a similar malady, that is, relatively high insertion losses and hence relatively high lasing thresholds.
More specifically, in the ring topology, the single pass gain through the active medium has to exceed the optical insertion loss of the other intracavity components (i.e., the modulator, filter, isolator, etc.) in order to produce lasing. Similarly, in the case of the FP configuration the double pass gain of the active medium has to offset the double pass insertion loss produced by the same type of intracavity components.
One approach to alleviating this problem is to arrange for the laser radiation to make a double pass through its active medium for every single pass through the other intracavity components, thereby reducing the insertion loss introduced by those components. Such a design produces better performance in terms of lower lasing threshold and higher pump conversion efficiency. These indicia translate into higher power, shorter duration, and more stable pulses in both Q-switched and mode-locked lasers. See, for example, the mode-locked Er-fiber laser described by T. F. Carruthers et al. in Optic Letters, Vol. 21, No. 23, pp. 1927-1929, Dec. 1, 1996. In the Carruthers laser one mirror of the FP design is formed by a conventional Faraday rotator, but the other mirror comprises a polarization-maintaining (PM) fiber loop which is coupled to the active medium by a polarization splitter-combiner (PSC). A polarization dependent modulator is located in the loop along with a polarization-rotating splice, an isolator, and an output coupler. However, this design suffers from several disadvantages. First, in order that light in the Carruthers loop be coupled back through the PSC to the gain medium, the polarization must be rotated by 90.degree.; hence the loop includes a polarization-rotating splice. Second, the loop contains an isolator to provide isolation to the input side of the modulator. (To enhance performance the design might in practice very well include a second isolator on the output side of the modulator.) Third, the behavior of the modulator and the pulse quality (i.e., the shape and/or timing) may be adversely affected by the presence of the multiplicity of other components (i.e., the isolator, coupler, polarization-rotating splice) in the loop. Lastly, the presence of these additional elements in the loop increases the complexity of the design as well as the number of splices, and hence the total insertion loss.
Thus, a need remains in the art for simpler FP laser design that reduces insertion loss without the disadvantages attendant prior art FP designs.